The present invention relates to optical sources for use in optical fiber communications or the like, and more particularly to an optical source that uses an in-fiber grating resonator to maintain a stable frequency.
Various communication systems, such as cable television (CATV) systems, currently distribute information signals via coaxial cable. The replacement of coaxial cable with optical fiber transmission lines in such communication systems has become a high priority. Production single mode fiber can support virtually unlimited bandwidth and has low attenuation. Accordingly, a fiber optic distribution system or a fiber-coax cable hybrid would provide substantially increased performance at a competitive cost as compared to prior art coaxial cable systems.
Frequency stable lasers are required as master oscillators in communication systems which have large numbers of lasers. Such lasers also have application to other fields, such as high resolution spectroscopy and gravity wave detection. Attempts have been made to provide frequency stable lasers in the past. For example, helium-neon, dye, and argonion lasers have been successfully stabilized for various applications. Such stabilized lasers are typically discrete element systems having wideband frequency noise, requiring complex locking servos. The use of reference interferometers has also been made to stabilize diode-laser-pumped Nd:YAG lasers. External elements are required in such lasers to force single axial mode operation and provide isolation against optical feedback.
The frequency stabilization of two diode-laser-pumped ring lasers that are independently locked to the same high finesse interferometer has been proposed by T. Day, E. K. Gustafson, and R. L. Byer in "Active Frequency Stabilization of a 1.062-.mu.m, Nd:GGG, Diode-Laser-Pumped Nonplanar Ring Oscillator to Less than 3 Hz of Relative Linewidth," Optics Letters, Vol. 15, No. 4, pp. 221-223 (1990). This article discloses the measurement of relative frequency stability by locking the lasers one free spectral range apart and observing the heterodyne beat note. In a subsequent paper, a method for locking an optical source to a resonance in an external Fabry-Perot cavity was described. T. Day and R. A. Marsland, "Optical Generation of a Frequency Stable mm-Wave Radiation Using Diode Laser Pumped Nd:YAG Lasers for Visible and IR Photo Detector Calibration," Optical Society of America, Nov. 3-8, 1991. The use of an external Fabry-Perot cavity adds complexity and cost to the system design. Additionally, Fabry-Perot cavities as taught by Day, et al. have many resonant frequencies, making frequency stabilization more difficult since a system may lock to the wrong resonance.
It would be advantageous to provide a stabilized optical frequency source that is reliable, easy to manufacture, and very low in cost. Such a source would have particular application in the communication of information signals over an optical fiber network. The low cost of such a source would be of particular importance in consumer oriented communication systems, such as CATV systems. It would be still further advantageous to provide a frequency stabilized optical signal source with its optical frequency locked to an in-fiber grating resonator having a single resonance.
The present invention provides a frequency stabilized optical signal source having the aforementioned advantages.